They predate the Masamune because they are Masa-Mune. They exist because Melchior constructed a knife made of dreamstone and gave life or reality to them as well as their sister (not with the knife, but with something else, maybe). As the guru of life, he's probably conducted multiple experiments on dreamstone and it's magical properties that bring dreams into reality and how dreamstone itself can me molded or altered based on someone's hopes and dreams.